Mechanical shock resistant MEMS accelerometer arrangement, associated method, apparatus and system

ABSTRACT

An accelerometer arrangement and method are described for determining accelerations of an inground tool. First and second triaxial accelerometers are supported such that a normal sensing axis of the first triaxial accelerometer is at least generally orthogonal to the normal sensing axis of the second triaxial accelerometer for determining the accelerations along the three orthogonal axes based on a combination of sensing axis outputs from one or both of the triaxial accelerometers. A weaker sensing axis of one triaxial accelerometer can be supported at least approximately normal to a weaker sensing axis of another triaxial accelerometer such that the weaker axes are not used. The triaxial accelerometers can be supported such that one axis of one accelerometer can be redundant with respect to another axis of another accelerometer. One triaxial accelerometer can be mounted on a tilted plane with respect to another triaxial accelerometer.

RELATED APPLICATIONS

The present application claims priority from U.S. Provisional PatentApplication Ser. No. 62/019,887, filed on Jul. 2, 2014, and U.S. U.S.Provisional Patent Application Ser. No. 62/021,618, filed on Jul. 7,2014, each of which is hereby incorporated by reference in its entirety.

BACKGROUND

The present application is generally related to MEMS accelerometers and,more particularly, to a MEMS accelerometer package having enhancedresistance to mechanical shock as well as an associated method,apparatus and system.

Accelerometers have become ubiquitous in modern electronic devices. Inthis regard, the measurement of acceleration along three orthogonal axescan provide for essentially complete characterization of deviceorientation. An accelerometer that is configured for measuringaccelerations along three orthogonal axes in a single, convenientpackage is often referred to as a triaxial accelerometer.

Many modern electronic systems employ accelerometers for a wide range ofpurposes. As a general example, the operational state or physicalorientation of an associated device can be characterized. As anothermore device specific example, an accelerometer can be used to detectthat a hard disk drive is in a state of free fall such that theread/write heads of the drive can be parked in anticipation ofmechanical shock upon impending impact. As still another device specificexample, modern cellular smart phones typically include an accelerometerto determine the orientation of the phone for display orientationmanagement and for use by a wide range of applications that can beinstalled on the smart phone. It is noted that the need foraccelerometers suited for consumer-grade electronics such as cellularsmart phones has driven the development of MEMS triaxial accelerometersthat are typically low-cost.

As still another device specific example, an accelerometer can be usedas part of a transmitter that is carried by an inground tool in ahorizontal directional drilling system for monitoring the orientationand movement of the inground tool. Such monitoring can facilitatesteering as well as monitoring the position of the inground tool. Aswill be described in detail below, Applicants recognize that the use ofconsumer-grade low-cost accelerometers in a device that subjects theaccelerometer to a mechanical shock and vibration environment can leadto failures of these devices. While the overall failure rate hashistorically not been high, any premature failure of a transmitter canresult in significant problems, including idling a crew and equipmentwhile a new transmitter is obtained, missing deadlines, as well as thecost involved with purchasing a new transmitter. To date, the industryhas continued to use these accelerometers for lack of an identified,practical alternative.

The present application brings to light a new approach which providesfor the use of low-cost consumer-grade accelerometers in a highmechanical shock and vibration environment in a way which enhancesreliability.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In general, an accelerometer arrangement and method are described fordetermining accelerations of an inground tool along three orthogonalaxes during an inground operation that exposes the accelerometerarrangement to a mechanical shock and vibration environment. In oneaspect of the disclosure, the accelerometer arrangement includes a firstMEMS triaxial accelerometer and a second MEMS triaxial accelerometer,each of which includes a set of three orthogonally arrangedaccelerometer sensing axes including a pair of in-plane sensing axes anda normal sensing axis such that the normal sensing axis is subject to ahigher rate of failure responsive to mechanical shock and vibration thanthe in-plane sensing axes. A support structure supports the first andsecond triaxial accelerometers such that the normal sensing axis of thefirst triaxial accelerometer is at least generally orthogonal to thenormal sensing axis of the second triaxial accelerometer. A processordetermines the accelerations along the three orthogonal axes based on acombination of sensing axis outputs from one or both of the first andsecond triaxial accelerometers.

In another aspect of the disclosure, the accelerometer arrangementincludes a first MEMS triaxial accelerometer and a second MEMS triaxialaccelerometer, each of which includes a weaker sensing axis that is moresusceptible to mechanical shock and vibration than the other two sensingaxes. A support structure supports the first and second triaxialaccelerometers such that the weaker sensing axis of the first triaxialaccelerometer is at least approximately normal to the weaker sensingaxis of the second triaxial accelerometer. A processor determines theaccelerations along the three orthogonal axes based on a combination ofsensing axis outputs from the first and second triaxial accelerometerswithout using the weaker sensing axis of each of the first and secondtriaxial accelerometers.

In still another aspect of the disclosure, the accelerometer arrangementincludes a first accelerometer package and a second accelerometerpackage, each of which includes one or more sensing axes such that thefirst and second accelerometer packages collectively provide a total ofat least four sensing axes for sensing along the three orthogonal axes.A support structure supports the first and second accelerometers suchthat at least one sensing axis of the first accelerometer package isredundant with respect to at least one sensing axis of the secondaccelerometer package. A processor is configured to select a combinationof three sensing axes from the total number of sensing axes fordetermining the accelerations along the three orthogonal axes.

In yet another embodiment of the present disclosure, the accelerometerarrangement includes a first MEMS triaxial accelerometer and a secondMEMS triaxial accelerometer. A support structure supports the first andsecond triaxial accelerometers such that the first triaxialaccelerometer is supported on a first plane that forms an angle of atleast approximately 45 degrees with respect to a second plane thatsupports the second triaxial accelerometer. A processor determines theaccelerations along the three orthogonal axes based on a combination ofsensing axis outputs from the first and second triaxial accelerometers.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Example embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be illustrative rather than limiting.

FIG. 1 is a diagrammatic view, in elevation, of a transmitter, accordingto the present disclosure, which utilizes dual triaxial MEMSaccelerometers.

FIG. 2 is a diagrammatic perspective view of an embodiment of theaccelerometer arrangement of the transmitter of FIG. 1.

FIG. 3 is a schematic diagram illustrating an embodiment of thetransmitter of FIG. 1.

FIG. 4 is a flow diagram that illustrates an embodiment for theoperation of the transmitter of FIG. 1.

FIGS. 5 and 6 are diagrammatic views, in perspective, of an embodimentof the accelerometer arrangement of FIG. 1.

FIG. 7 is a flow diagram illustrating an embodiment of a method foroperating a transmitter in accordance with the present application basedon a priority table of sensing axis combinations derived from two ormore accelerometers.

FIG. 8 is a diagrammatic illustration, in a perspective view, of anotherembodiment for supporting dual accelerometers in accordance with thepresent disclosure, wherein the accelerometers are supported on planesthat are non-normal with respect to one another.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles taught herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown, but is to be accorded the widest scopeconsistent with the principles and features described herein includingmodifications and equivalents, as defined within the scope of theappended claims. It is noted that the drawings are not to scale and arediagrammatic in nature in a way that is thought to best illustratefeatures of interest. Descriptive terminology may be used with respectto these descriptions, however, this terminology has been adopted withthe intent of facilitating the reader's understanding and is notintended as being limiting. Further, the figures are not to scale forpurposes of illustrative clarity.

Turning now to the figures wherein like components are indicated by likereference numbers throughout the various figures, attention isimmediately directed to FIG. 1 which is an elevational view thatdiagrammatically illustrates an embodiment of a transmitter, generallyindicated by the reference number 10, for use in a horizontaldirectional drilling system. Transmitter 10 is supported by an ingroundtool 14 such as, for example, a boring tool for performing a drillingoperation or a tension monitoring arrangement used with a back-reamingtool for performing a pull-back operation. As will become apparent, thepresent disclosure is relevant with respect to any application whichsubjects certain components to a mechanical shock and vibrationenvironment.

Transmitter 10 includes a processor 20 in communication with atransmitter section 22 that is coupled to an antenna 24 for emitting asignal 28 such as, for example, a locating signal which can be a dipolesignal. The transmitter can include an elongation axis 30 that can atleast generally align with an elongation axis of inground tool 14 whenthe transmitter is installed therein. It is noted that inter-componentcabling is not shown in FIG. 1 for purposes of illustrative clarity, butis understood to be present. In another embodiment, transmitter section10 can comprise a transceiver for also receiving an incomingelectromagnetic signal. In still another embodiment, above groundcommunication can be implemented by using a drill string (not shown)that is attached to inground tool 14 as an electrical conductor asdescribed, for example, in U.S. Published Application no. 2013/0176139which is hereby incorporated by reference in its entirety. It should beappreciated that the teachings herein remain applicable irrespective ofthe particular communication pathway or pathways that are implemented.Any appropriate combination of sensors can be provided as part of thetransmitter such as, for example, a pressure sensor 32, a temperaturesensor 36 and an accelerometer arrangement 40. Data collected from thesevarious sensors by processor 20 can be transmitted, for example, bymodulating signal 28.

Referring to FIG. 2 in conjunction with FIG. 1, in one embodiment,accelerometer arrangement 40 comprises a MEMS accelerometer arrangementthat is supported within transmitter 10. The MEMS accelerometerarrangement includes a first triaxial MEMS accelerometer 44 a and asecond triaxial MEMS accelerometer 44 b. It is noted that theseaccelerometers may be referred to collectively by the reference number44. FIG. 2 is an enlarged diagrammatic view, in perspective, of MEMSaccelerometer package 40. First triaxial MEMS accelerometer 44 aincludes orthogonal sensing axes x₁, y₁ and z₁, while second triaxialMEMS accelerometer 44 b includes orthogonal sensing axes x₂, y₂ and z₂.It is noted that axes x₂ and z₁ can be at least approximately aligned orparallel with elongation axis 30, although this is not required. Thetriaxial accelerometers can be supported and electrically connected inany suitable manner. In the present example, a first printed circuitboard 50 supports and electrically connects triaxial MEMS accelerometer44 a while a second printed circuit board 52 supports and electricallyconnects second triaxial MEMS accelerometer 44 b. Generally, the samepart number can be used for both triaxial MEMS accelerometers, althoughthis is not required. The triaxial MEMS accelerometers can beinexpensive, consumer-grade MEMS accelerometers such as, for example,those that are found in cellular telephones. One non-limiting example ofsuch a consumer-grade triaxial accelerometer is the MMA 8451Qmanufactured by Freescale Semiconductor. In the present embodiment,accelerometers 44 are configured with an I2C interface such thatprocessor 20 accesses readings along each axis by reading from specificmemory locations within each triaxial accelerometer, although anysuitable form of interface, including analog, can be utilized.

Triaxial MEMS accelerometers have become the most widely available, andin many cases least expensive, accelerometers available in the market.These accelerometers have become pervasive in applications ranging fromcellular telephones to toys. However, underground drilling can subjectaccelerometers to higher and/or more prolonged levels of mechanicalshock that may in some circumstances exceed the thresholds for whichthese components are typically designed. In particular, Applicantsrecognize that the z-axis, which can be referred to as a normal axis ora weaker axis, of triaxial MEMS accelerometers tend to fail more oftenthan other axes, which can be referred to as the in-plane axes, whenexposed to extreme shock conditions, thereby limiting the overallperformance of the component. Manufacturers of locating systems usethese components in spite of this limitation in the absence of a moresuitable alternative. The teachings that are brought to light hereincompensate for this limitation, providing the potential for asignificantly higher level of reliability when these accelerometers areused in underground drilling applications or other conditions of extrememechanical shock.

As best seen in the perspective view of FIG. 2, MEMS accelerometer 44 ais mounted orthogonal to MEMS accelerometer 44 b such that the z₁ axisof accelerometer 44 a is at least approximately orthogonal to the z₂axis of accelerometer 44 b and the x₁, y₁ plane is at leastapproximately orthogonal to the x₂, y₂ plane. It is noted that the x andy sensing axes of a given accelerometer may be referred to as thein-plane axes since these axes define a plane that is at least generallyparallel to the surface on which the triaxial accelerometer package issupported, while the z axis may be referred to as a normal axis. In anembodiment, accelerometer readings for three orthogonally disposed axescan be obtained based on sensing along axis x₁ of accelerometer 44 a aswell as along axes x₂ and y₂ of accelerometer 44 b such that the x₁accelerometer readings are used in place of z₂ readings fromaccelerometer 44 b. In another embodiment, accelerometer readings can beobtained based on sensing along axes x₁ and y₁ of accelerometer 44 a andalong axis x₂ of accelerometer 44 b such that the x₂ accelerometerreadings are used in place of z₁ readings from accelerometer 44 a. Ineither of these embodiments, no sensor readings are needed from the zaxes of the dual accelerometers. Stated in another way, two triaxialMEMS accelerometer packages can be mounted normal or orthogonal to oneanother, thereby allowing the normal accelerometer in one package to bereplaced by an in-plane accelerometer in the other package. Thissolution provides for more than the simple redundancy of using multipleaccelerometers in the standard manner (i.e. using all three axes), sincethis would still expose the device to potential failure of the z-axis.By contrast, Applicant's configuration is specifically designed toeliminate the recognized weakest link in these components—namely, thez-axis.

Referring to FIG. 3, a schematic diagram of an embodiment of transmitter10 is illustrated. Descriptions of like components will not be repeatedfor purposes of brevity. In the present embodiment, an I2C interface 100connects triaxial MEMS accelerometers 44 a and 44 b to CPU 20. Pressuresensor 32 and temperature sensor 36 can be interfaced to processor 20using an analog to digital converter 104, if either sensor is an analogsensor.

FIG. 4 is a flow diagram that illustrates one embodiment of a method,generally indicated by the reference number 200, for the operation oftransmitter 10 in accordance with the present disclosure. The methodbegins at start 204 and proceeds to 208 which reads axis x₁ ofaccelerometer 44 a. At 210, axis x₂ of accelerometer 44 b is readfollowed by reading axis y₂ of accelerometer 44 b at 212. The variousaccelerometer axes can be read individually, in any suitable order orcombination, based on the parameter or parameters that are beingdetermined. Such parameters include, by way of non-limiting example,pitch and roll of inground tool 14. At 216, the accelerometer readingsare used to perform determinations such as, for example, pitch and/orroll orientation parameters.

FIGS. 5 and 6 are diagrammatic views, in perspective of an embodiment ofaccelerometer arrangement 40, shown here for further illustrativepurposes.

It should be appreciated that in addition to providing a robustaccelerometer arrangement using low-cost triaxial MEMS accelerometers,still further benefits are provided by the foregoing embodiments. Forexample, redundancy can be provided with respect to accelerometerreadings. Referring to FIG. 2, in an embodiment that uses axes x₁, x₂and y₂, axis y₁ of accelerometer 44 a is unused and can be used in placeof axis y₂ of accelerometer 44 b, for example, if a failure of y₂ isexperienced.

The teachings that have been brought to light above can readily beapplied to other embodiments that are considered as being within thescope of the present application so long as a given embodiment practicesavoiding the use of a weaker axis. For example, in one embodiment, adual accelerometer arrangement can comprise a pair of dual axisaccelerometers such that the normal, z, or weaker axis is not present ineither accelerometer and the dual accelerometers are supported at leastapproximately normal or orthogonal to one another such that an in-planeaxis of one of the dual axis accelerometers serves as a z or normalaxis. In another embodiment, a triaxial accelerometer can be paired witha dual axis accelerometer such that an in-plane axis of the dual axisaccelerometer is supported to serve in place of the weaker, z, or normalaxis of the triaxial accelerometer. In still another embodiment, atriaxial accelerometer can be paired with a single axis accelerometersuch that the single axis accelerometer is supported to serve in placeof the weaker, z, or normal axis of the triaxial accelerometer. In yetanother embodiment, a dual axis accelerometer having a pair of in-planeaccelerometers can be paired with a single axis accelerometer such thatthe single axis accelerometer is arranged at least approximately normalor orthogonal to the in-plane accelerometers of the dual axisaccelerometer.

While the foregoing teachings provide for a much hardier accelerometerpackage overall, additional robustness can be obtained from the twotri-axial packages, as will be described immediately hereinafter.

It should be appreciated that two tri-axial packages can provide eightcombinations of accelerometers that can serve as a single tri-axialaccelerometer with two accelerometers being available for each Cartesiandirection. Table 1 lists the combinations in light of the accelerometeraxes shown in FIG. 2. It is noted that a set of master coordinate axesX, Y, Z are shown in FIG. 2 such that the final column of Table 1indicates the sign associated with the respective master coordinate axisfor each sensing axis of a given combination.

TABLE 1 AVAILABLE COMBINATIONS OF AXES FOR DUAL TRIAXIAL ACCELEROMETERSCombination Axes of Master Pitch Roll no. Combination Coordinate Axes Øβ  1* x₂, y₂, x₁ X, Y, Z sin⁻¹ x₂ tan⁻¹(x₁/y₂) 2 x₂, y₁, z₂ X, −Y, Zsin⁻¹ x₂ tan⁻¹(z₂/y₁) 3 z₁, y₂, z₂ X, Y, Z sin⁻¹ z₁ tan⁻¹(z₂/y₂)  4* x₂,y₁, x₁ X, −Y, Z sin⁻¹ x₂ tan⁻¹(x₁/y₁) 5 z₁, y₂, x₁ X, Y, Z sin⁻¹ z₁tan⁻¹(x₁/y₂) 6 z₁, y₁, z₂ X, −Y, Z sin⁻¹ z₁ tan⁻¹(z₂/y₁) 7 z₁, y₁, x₁ X,−Y, Z sin⁻¹ z₁ tan⁻¹(x₁/y₁) 8 x₂, y₂, z₂ X, Y, Z sin⁻¹ x₂ tan⁻¹(z₂/y₂)*= enhanced reliability

As denoted by asterisks in Table 1, there are two combinations, 1 and 4,that provide the hardiest arrangement by eliminating all use of thenormal accelerometer in both accelerometer packages.

For combinations 1, 2, 4 and 8, pitch, which is designated as Ø, isgiven as:Ø=sin⁻¹ x ₂  EQN (1)

While roll, designated as β, for combinations 1 and 5 is given, by wayof example, as:

$\begin{matrix}{\beta = {\tan^{- 1}\left\lbrack \frac{x_{1}}{y_{2}} \right\rbrack}} & {{EQN}\mspace{14mu}(2)}\end{matrix}$

And roll for combinations 2 and 6 is given, by way of example, as:

$\begin{matrix}{\beta = {\tan^{- 1}\left\lbrack \frac{z_{2}}{y_{1}} \right\rbrack}} & {{EQN}\mspace{14mu}(3)}\end{matrix}$

Applicants recognize that the proper functionality of each combinationin Table 1 can be verified by summing the squares of the threeaccelerations for each combination. The sum should equal gravitationalacceleration, g, squared. Representing the three accelerometer readingsfor each combination generically using the variables a, b and c:g ² =a ² +b ² +c ²  EQN (4)

In actual practice, a range limit can be placed on the sum of thesquares of Equation 4 to account for accuracy of the accelerometers andother measurement errors such that the accelerometers associated with aparticular sum of the Equation 4 can be deemed as operating correctly solong as the sum falls between g_(min) ² and g_(max) ². Suitable valuesfor g_(min) ² and g_(max) ², by way of non-limiting example, are atleast approximately 0.958 g² and 1.05 g², respectively, or a change of+/−5 percent from 1 g.

In an embodiment, an ordered list of accelerometer combinations, havingthe most reliable combinations at the top of the list as a preference,can be utilized to determine which accelerometer combination to use.Table 2 represents one embodiment of such an ordered list wherein thecombination numbers from Table 1 are set forth.

TABLE 2 PRIORITY TABLE FOR DUAL TRIAXIAL ACCELEROMETERS PriorityCombination Axes of Order no. Combination 1  1* x₂, y₂, x₁ 2  4* x₂, y₁,x₁ 3 2 x₂, y₁, z₂ 4 3 z₁, y₂, z₂ 5 5 z₁, y₂, x₁ 6 6 z₁, y₁, z₂ 7 7 z₁,y₁, x₁ 8 8 x₂, y₂, z₂ *= enhanced reliability

It should be appreciated that the use of a priority table such as Table2 does not require the use of one or more accelerometers having a weakeraxis. Priority assignments can be made in accordance with any sort ofconcern that relates to reliability. By way of non-limiting example,such concerns can derive from the reliability of physical mounting,supporting electrical connections, environmental exposure, and history.In some embodiments, the priority table can be used even when theaccelerometer axis combinations are thought to all exhibit at leastgenerally the same reliability in order to provide an overall level ofreliability that is submitted to be heretofore unseen.

Referring to FIG. 7, an embodiment of a method is illustrated foroperating transmitter 10 based on a priority table such as Table 2 isgenerally indicated by the reference number 200. Method 200 begins atstart 204, for example, when the transmitter and accelerometers arefirst turned on and proceeds to 208. This latter step sets each of apriority counter and a loop counter to a value of 1. The purpose of theloop counter will be brought to light at an appropriate pointhereinafter. At 210, a sum of the squares of the accelerometers for thefirst or highest order combination of accelerometers in Table 2 isproduced. At 214, the sum of the squares value is tested against gmin²and gmax², if the value is within range, operation is routed to 218 suchthat the current combination of accelerometers is used for normaloperation. During normal operation, the selected accelerometercombination can be periodically monitored and/or tested at 220 forfailure, for example, based on Equation 4 and/or any other suitablefactors. If no failure is detected, normal operation resumes at 224. Ifan accelerometer failure is detected at 220, operation returns to 208such that the procedure begins anew.

Returning to the discussion of 214, if the sum of the squares is out ofrange, operation proceeds to 230 which increments the value of thepriority order counter by 1. At 234, the value of the priority ordercounter is tested against the total number of available accelerometercombinations in Table 2. If the current value of the priority ordercounter does not exceed the total number of available combinations,operation returns to 210 and proceeds therefrom. Otherwise, operationproceeds to 238 which tests the current value of the loop counteragainst a loop count limit. The purpose of the loop counter relates tothe potential for MEMS accelerometers to become temporarily stuck due tostatic charge forces. Accordingly, it is not necessary to stop theaccelerometer selection procedure based on reaching the bottom of thepriority table list. Instead, the priority table list can be loopedthrough repeatedly some number of times before the accelerometer packageis declared unusable or the selection process can continue indefinitelyin hopes that the accelerometers become functional. As part of the looparchitecture, it should be appreciated that every available combinationof sensing axes can be tested or re-tested including a combination thatinvoked the test procedure in the first instance, for example, based ondetection by step 220. In this way, a previously failed combination thatsubsequently becomes functional can be placed back into service. It isnoted that the test of step 238 and a loop architecture is not arequirement. In an embodiment that does not employ a loop count, step214 can notify the operator that accelerometer testing is beingperformed each time this step is entered. If the loop count is notexceeded at 238, operation proceeds to 240 which increments the loopcount and sets the priority order counter to 1. Operation then returnsto 210 and proceeds therefrom. On the other hand, if 238 determines thatthe loop count exceeds a loop count limit, which can be established, forexample, by the manufacturer, a warning can be issued to the operator at244.

The method and associated apparatus described above can readily be usedwith additional accelerometer packages having any suitable number ofsensing axes and/or a single sensing axis for even more redundancy.Further, the procedures of FIGS. 4 and 7 are not limited to tri-axialaccelerometer packages and the accelerometer packages are not requiredto be normally mounted with respect to one another so long as threeCartesian acceleration directions can be resolved from theaccelerometers selected, as will be described in detail immediatelyhereinafter.

Attention is now directed to FIG. 8 which is a diagrammaticillustration, in a perspective view, of another embodiment forsupporting dual accelerometers in accordance with the presentdisclosure. It is noted that the accelerometer axes are shownindependent of physical packaging and the axes are indicated usingreference designations that are taken from FIGS. 1 and 2. First circuitboard 50 and second circuit board 52 are shown as planes for purposes ofdiagrammatic simplicity while accelerometers 44 a and 44 b areconsidered as being located at the origins of their respectivecoordinate axes. In the present embodiment, second circuit board 52 issupported at an acute angle β relative to first circuit board 52. Theangle β can have any suitable value. In one embodiment, β can be atleast approximately 45°. The x₂ axis can be at least approximatelyaligned or parallel with the elongation axis of the transmitter, as seenin FIG. 1, although this is not a requirement. The in-plane axes, x₁ andy₁ of accelerometer 44 a in FIG. 1 remain at least approximatelyparallel to first circuit board 50 but have been rotated by an angle.The z₁ axis is at least approximately orthogonal to the plane of circuitboard 50. In the present embodiment, angle α is at least approximatelyequal to 45°. In other embodiments, any suitable angle can be utilizedfor α. For example, α can be in the range from 20° to 160° which willallow for a sufficient projection onto axes of interest.

Still referring to FIG. 8, it should be appreciated that angles α and βcan be determined at the time of manufacture and/or determined based ona calibration procedure that is performed, for example, when thetransmitter and accelerometer arrangement are supported in a knownphysical orientation. The calibration procedure can orient thetransmitter in six cardinal orientations based on three orthogonal axesthat can be referenced to the housing of the transmitter. These cardinalorientations can correspond to roll positions of 0°, 90°, 180° and 270°with a pitch angle of 0°, and pitch angles of +/−90°. In this way,angles α and β can be determined as well as any angular variation ofaxis x₂ from the elongation axis of the transmitter. The orientation ofeach axis can be characterized in a well-known manner, for example,based on Euler angles that utilize the transmitter elongation axis and azero roll orientation, as references. Based on the described values forangles α and β, each axis of accelerometer 44 a is skewed or exhibits anangular offset with respect to every axis of accelerometer 44 b. As willbe seen, the accelerometer arrangement depicted in FIG. 8, andvariations thereof, provide a significant number of combinations of axesand flexibility for purposes of measuring roll and pitch orientation.

Table 3 sets forth the combinations of axes that can be used for rolland pitch orientation in accordance with the embodiment of FIG. 8. It isnoted that for each combination, two axes are used for detecting theroll orientation and a different axis is used for detecting the pitchorientation. For a particular combination, the axes used for detectionof roll orientation are designated using an “R” and the axis that isused for detection of pitch orientation is designated as “P”. As listedin the final column of the table, the pitch measurement can be sensitiveto angle β or angles β and β when the axis that is used for pitchmeasurement is skewed with respect to axis x₂, which is assumed forpurposes of the present example to be parallel to the elongation axis ofthe transmitter. Since axis x₂ is assumed to be parallel to theelongation axis, no such sensitivity is exhibited (designated as “N/A”in the table) with respect to pitch measurement using x₂.

TABLE 3 ACCELEROMETER AXIS COMBINATIONS FOR PITCH AND ROLL Pitchsensitivity Roll proportional Combination x₁ y₁ z₁ x₂ y₂ z₂ to: 1a P R RN/A 1b P R R sin β 1c P R R cos β, sin α 1d P R R cos β, cos α 2a R P RN/A 2b R P R cos β, cos α 2c R P R sin β 3a R P R N/A 3b R P R sin β 4aR P R N/A 4b R P R cos β, cos α 4c R P R sin β 5a R P R N/A 5b R P R sinβ 5c P R R cos β, sin α

Accordingly, fifteen different combinations are available. It should beappreciated that these combinations can be prioritized. For example,combinations that rely on either z₁ or z₂ can be assigned a relativelylower priority than combinations that do not rely on these axes.Combinations that rely on z₁ and z₂ can be assigned still lowerpriority. Applicants submit that the wide range of combinations of axesin Table 1 can provide for significant immunity with respect to thefailure of one or more accelerometer axes in terms of pitch and rollmeasurement. It is noted that 14 out of the 15 combinations given inTable 3 utilize outputs from both triaxial accelerometers.

Still referring to FIG. 8, additional combinations can be used whichutilize all three axes of each accelerometer. In these embodiments, theorthogonal x, y and z accelerometer measurements taken by eitheraccelerometer 44 a or 44 b can be resolved onto reference axes of thetransmitter, for example, onto the transmitter elongation axis and anaxis that corresponds to a zero roll orientation. In the example of FIG.8, the transmitter elongation axis is additionally designated as Gx andcorresponds to axis x₂ under the assumption that x₂ is at leastapproximately parallel thereto. A roll orientation reference axis Gycorresponds to axis y₂, assuming that the transmitter is oriented at aroll reference position of zero degrees, and a reference axis Gz that isorthogonal to both Gx and Gy. With these values in hand and in oneembodiment, the roll orientation can be determined based on:Roll=a tan 2(Gy,Gz)  EQN (5)

It should be appreciated that the function a tan 2 is an arctangentfunction with two arguments which returns the appropriate quadrant forthe roll angle that is determined.

In another embodiment, the roll orientation can be determined based on

$\begin{matrix}{{Roll} = {\arctan\left\lbrack \frac{Gy}{\sqrt{{Gx}^{2} + {Gz}^{2}}} \right\rbrack}} & {{EQN}\mspace{14mu}(6)}\end{matrix}$

Accordingly, even more flexibility is provided based on Equations 5 and6 with respect to the ability to determine roll orientation.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form or formsdisclosed, and other modifications and variations may be possible inlight of the above teachings. Accordingly, those of skill in the artwill recognize certain modifications, permutations, additions andsub-combinations of the embodiments described above.

What is claimed is:
 1. An accelerometer arrangement configured todetermine accelerations of an inground tool along three orthogonal axesduring an inground operation that exposes the accelerometer arrangementto a mechanical shock and vibration environment, said accelerometerarrangement comprising: a first MEMS triaxial accelerometer and a secondMEMS triaxial accelerometer, each of which includes a set of threeorthogonally arranged accelerometer sensing axes including a pair ofin-plane sensing axes and a normal sensing axis such that the normalsensing axis is subject to a higher rate of failure responsive tomechanical shock and vibration than the in-plane sensing axes; a supportstructure configured to support the first and second triaxialaccelerometers such that the normal sensing axis of the first triaxialaccelerometer is at least generally orthogonal to the normal sensingaxis of the second triaxial accelerometer; and a processor configured todetermine the accelerations along said three orthogonal axes based on acombination of sensing axis outputs from the first and second triaxialaccelerometers without using normal sensing axis outputs of each of thefirst and second triaxial accelerometers.
 2. The accelerometerarrangement of claim 1 supported within a transmitter that is carried bythe inground tool.
 3. An accelerometer arrangement configured todetermine accelerations of an inground tool along three orthogonal axesduring an inground operation when supported within a transmitter that iscarried by the inground tool which inground operation exposes theaccelerometer arrangement to a mechanical shock and vibrationenvironment, and the transmitter includes an elongation axis, saidaccelerometer arrangement comprising: a first MEMS triaxialaccelerometer and a second MEMS triaxial accelerometer, each of whichincludes a set of three orthogonally arranged accelerometer sensing axesincluding a pair of in-plane sensing axes and a normal sensing axis suchthat the normal sensing axis is subject to a higher rate of failureresponsive to mechanical shock and vibration than the in-plane sensingaxes; a support structure configured to support the first and secondtriaxial accelerometers such that the normal sensing axis of the firsttriaxial accelerometer is at least generally orthogonal to the normalsensing axis of the second triaxial accelerometer and one sensing axisof the first triaxial accelerometer and another sensing axis of thesecond triaxial accelerometer are at least generally parallel to theelongation axis; and a processor configured to determine theaccelerations along said three orthogonal axes based on a combination ofsensing axis outputs from the first and second triaxial accelerometerswithout using normal sensing axis outputs of each of the first andsecond triaxial accelerometers.
 4. The accelerometer arrangement ofclaim 2 wherein at least one in-plane sensing axis of the first andsecond triaxial accelerometers is arranged for sensing a pitchorientation of the transmitter.
 5. The accelerometer arrangement ofclaim 2 wherein the pair of in-plane sensing axes of one of the firstand second tri-axial accelerometers is supported for detecting a rollorientation of the transmitter.
 6. The accelerometer arrangement ofclaim 1 wherein said support structure includes a first printed circuitboard that supports the first triaxial accelerometer and a secondprinted circuit board that supports the second triaxial accelerometer.7. The accelerometer arrangement of claim 6 wherein the second printedcircuit board is supported by the first printed circuit board at leastgenerally orthogonal thereto.
 8. An accelerometer arrangement configuredto determine accelerations of an inground tool along three orthogonalaxes during an inground operation that exposes the accelerometerarrangement to a mechanical shock and vibration environment, saidaccelerometer arrangement comprising: a first MEMS triaxialaccelerometer and a second MEMS triaxial accelerometer, each of whichincludes a set of three orthogonally arranged accelerometer sensing axesincluding a pair of in-plane sensing axes and a normal sensing axis suchthat the normal sensing axis is subject to a higher rate of failureresponsive to mechanical shock and vibration than the in-plane sensingaxes: a support structure for supporting the first and second triaxialaccelerometers such that the normal sensing axis of the first triaxialaccelerometer is at least generally orthogonal to the normal sensingaxis of the second triaxial accelerometer; and a processor configured todetermine the accelerations along said three orthogonal axes based on acombination of sensing axis outputs from the first and second triaxialaccelerometers and configured to select the combination of sensing axisoutputs based on a priority table.
 9. The accelerometer arrangement ofclaim 8 wherein the first and second triaxial accelerometers provide aset of sensing axis combinations and said priority table is arrangedaccording to a reliability of at least some of the combinations in theset of sensing axis combinations.
 10. The accelerometer arrangement ofclaim 9 wherein a first combination and a second combination areassigned as a first priority and a second priority in the priority tableand each of the first combination and the second combination exclude thenormal sensing axis of the first and second triaxial accelerometers. 11.The accelerometer arrangement of claim 8 wherein the processor isconfigured to detect a failure of one or more sensing axes in thecombination and, responsive thereto, loop through the priority table tofind a usable combination of sensing axes from the set of sensing axiscombinations.
 12. The accelerometer arrangement of claim 11 wherein saidprocessor is configured to loop through the priority table a pluralityof times.
 13. The accelerometer arrangement of claim 12 wherein saidprocessor is configured to issue a warning responsive to looping throughthe priority table said plurality of times without identifying a usablecombination.
 14. The accelerometer arrangement of claim 11 wherein theaforerecited combination of sensing axis outputs is identified as afailed combination and the failed combination is re-tested as part oflooping through the priority table to find the usable combination. 15.The accelerometer arrangement of claim 14 wherein said processor isconfigured to place the failed combination back into service responsiveto detecting that the failed combination has become functional.
 16. Theaccelerometer arrangement of claim 11 wherein the processor detects saidfailure based on a sum of the squares of a set of three outputs for thecombination of sensing axes.
 17. A method for determining accelerationsof an inground tool along three orthogonal axes during an ingroundoperation that exposes an accelerometer arrangement to a mechanicalshock and vibration environment, said method comprising: supporting afirst MEMS triaxial accelerometer and a second MEMS triaxialaccelerometer, each of which includes a set of three orthogonallyarranged accelerometer sensing axes including a pair of in-plane sensingaxes and a normal sensing axis such that the normal sensing axis issubject to a higher rate of failure responsive to mechanical shock andvibration than the in-plane sensing axes, to arrange the normal sensingaxis of the first triaxial accelerometer at least generally orthogonalto the normal sensing axis of the second triaxial accelerometer; anddetermining the accelerations along said three orthogonal axes based ona combination of sensing axis outputs from the first and second triaxialaccelerometers without using normal sensing axis outputs of each of thefirst and second triaxial accelerometers.
 18. An accelerometerarrangement configured to determine accelerations of an inground toolalong three orthogonal axes during an inground operation that exposesthe accelerometer arrangement to mechanical shock and vibration, saidaccelerometer arrangement comprising: a first MEMS triaxialaccelerometer and a second MEMS triaxial accelerometer, each of whichincludes a weaker sensing axis that is more susceptible to mechanicalshock and vibration than the two other sensing axes and normal to theother two sensing axes; a support structure configured to support thefirst and second triaxial accelerometers such that the weaker sensingaxis of the first triaxial accelerometer is at least approximatelynormal to the weaker sensing axis of the second triaxial accelerometer;and a processor configured to determine the accelerations along saidthree orthogonal axes based on a combination of sensing axis outputsfrom the first and second triaxial accelerometers without using weakersensing axis outputs of each of the first and second triaxialaccelerometers.
 19. An accelerometer arrangement for determiningaccelerations of an inground tool along three orthogonal axes during aninground operation, said accelerometer arrangement comprising: a firstaccelerometer package and a second accelerometer package, each of whichincludes one or more sensing axes such that the first and secondaccelerometer packages collectively provide a total of at least foursensing axes for sensing along said three orthogonal axes and at leastone of the first accelerometer package and the second accelerometerpackage includes one sensing axis that is weaker than another sensingaxis of that accelerometer package in being more susceptible tomechanical shock and vibration; a support structure configured tosupport the first and second accelerometers such that at least onesensing axis of the first accelerometer package is redundant withrespect to at least one sensing axis of the second accelerometerpackage; and a processor that is configured to select a combination ofthree sensing axes from the total number of sensing axes to determinethe accelerations along said three orthogonal axes without using theweaker sensing axis for determining said accelerations.
 20. Theaccelerometer arrangement of claim 19 wherein said processor isconfigured to select the combination of sensing axis outputs based on apriority table.
 21. The accelerometer arrangement of claim 20 whereinthe first and second accelerometers provide a set of sensing axiscombinations based on the total number of sensing axes and said prioritytable is arranged according to a reliability of at least some of thecombinations in the set of sensing axis combinations.
 22. Theaccelerometer arrangement of claim 19 wherein the processor isconfigured to detect a failure of one or more sensing axes in thecombination and, responsive thereto, select a different combination ofsensing axes.
 23. The accelerometer arrangement of claim 22 wherein theprocessor is configured to detect said failure based on a sum of thesquares of a set of three outputs for the combination of sensing axes.